We’re told that a horse and an ass belong to different species because their offspring (a mule) is sterile. But what is the mule - a horse, an ass, neither, or both? Or is there a better definition of species that accounts for such cases?
The question seems to assums that all plants, animals, etc., must belong to a species. I would have though that sterile hybrids do not belong to a species.
That is correct. Mules, or other interspecific hybirds (whether sterile or fertile) are not considered to belong to any species. They would be described scientifically as Equus caballus x E. asinus.